Preemergence... atrazine would be the best herbicide for jimsonweed control, preemergence.  Whatever pre-emergence program you decide to use, be sure to have some atrazine in there in some form or fashion.

I pulled up a 2,4-D amine label.  2,4-D Amine can be applied 7-14 days before planting, or 3-5 days after planting.  Also, 2,4-D can be applied post-emergence to the sweet corn, up to 8" tall sweet corn.  If applying post-emergence, don't let the jimsonweed get too tall.  While jimsonweed is on the 2,4-D label, I don't think it is the best to control jimsonweed. So, if you spray with jimsonweed is, say, 2-3" tall, I think you will be fine. If you spray with jimsonweed is, say, 6" tall, you may have missed them already.  So, earlier is better.

Impact is labeled for sweet corn post-emergence.  Jimsonweed is on the "weeds controlled" list for Impact. You can also tank-mix some atrazine with the Impact - adding atrazine would help with post-emergence control of jimsonweed.  I think Impact is going to come in a container size that treats 20 (or 30) acres.

Laudis, as mentioned above, is labeled for sweet corn post-emergence.  Like Impact, Laudis can be tank-mixed with atrazine, and adding atrazine with the Laudis will help with jimsonweed control.  Laudis comes in a 1-gallon container, and 1 gallon is going to treat about 42-43 acres. 

I suggest checking into price of Laudis or Impact before you get to the point of needing them. Your neighbor might decide that springing for that Roundup-Ready sweet corn is a better alternative for him.

As a last resort, the "Santa Clause" weed control program works...Ho(e)... Ho(e)... Ho(e)... :)

edit: one more thought....  if the sweet corn plot is small - which I suspect it is - a lawn mower can be an effective weed control method when the sweet corn is up and growing.  A 22" push mower, with the deck set as high as possible, can be pushed between the rows and weeds can be mowed off.  I am presuming the sweet corn is planted in 30" rows, and not something quite narrower.